In today's optical communication systems, various studies and research have been conducted in order to develop technologies for large-capacity and long-distance transmission. Regarding transmission rate, modulation schemes superior in transmission characteristics and modulation efficiency have been researched. Concerning capacity enhancement, multiplexing schemes including wavelength division multiplexing (WDM), time division multiplexing, and polarization multiplexing have been researched. These studies and research are beneficial for dealing with continuously increasing information capacity.
Known modulation schemes include on-off keying (OOK) to modulate data making use of simple blinking of light signals, and phase shift keying (PSK) to modulate data by changing the phase of light signal. In addition, multilevel multi-phase modulation (m-PSK, n-ASK), which has high frequency usage efficiency, or orthogonal frequency division multiplexing (OFDM) are also attracting attention.
Besides these backbone communication schemes, optical frequency division multiplexing (referred to as “optical FDM”) has been proposed, which is applied mainly to a high-speed data distribution system at a transmission rate of several megabytes per second (Mb/s) to distribute video pictures or the like. Optical FDM is a transmission technique for multiplexing sub-carrier frequencies fi onto the frequency axis of a continuous wave (CW) carrier light. Optical FDM generally employs on-off keying to modulate the intensity of carrier light (wave) or phase keying to modulate the phase of light.
A technique for applying subcarrier multiplexing to a wavelength division multiplexing (WDM) network is also known. This technique is used to construct a broadband multimedia network. For example, superposing a sub-carrier multiplexed (SCM) control signal onto a baseband signal is proposed. (See Non-patent Document 1 listed below.) In this technique, a data-multiplexed baseband signal is used as a carrier wave. An intensity-modulated SCM signal is superposed onto the baseband signal. At a receiving site, the received signal is separated into the baseband signal and the SCM signal using a loop interferometer (filter). With this filtering technique, the SCM signal is completely separated from the baseband signal. In other words, the separated SCM signal has lost the carrier wave component, and is converted into a baseband signal without the carrier wave component. If two or more SCM signal components are multiplexed densely, the converted baseband signals interfere with each other and a desired channel cannot be extracted.